The invention concerns an integrated analog multiplexer with several multiplexer inputs, a multiplexer output, a switch device and a differential amplifier with an inverting and a non-inverting amplifier input as well as an amplifier output.
Such integrated analog multiplexers are used in the analog signal processing to selectively connect analog signals from different sources to a node. For that, complementary CMOS-transistor pairs that are powered by operational amplifiers are often used as a switch.
In modern technologies that are especially geared for digital performance, such integrated analog multiplexer do not function that good any more. For one, the reduced supply voltage (for example 1.2 Volt in 0.13 xcexcm processes), the scarcely reduced input voltage of the transistors and the larger substrate control effect lead to the result, that the forward resistances of the transistors becomes higher and the variation of the forward resistances increases in dependence of the voltage of the analog signals, so that undesired harmonic distortions occur during the through connect of the analog signals.
An attempt to prevent the variation of the forward resistance is, for example, to maintain the gate-source voltage constant with a boost-switch through which, at a voltage of the analog signal near the supply voltage, the gate voltage can increase over the supply voltage by approximately the input voltage. If now a discharged capacitor is to be charged over the drain port of the switching transistor, the gate-drain voltage is higher, especially at the beginning of the loading process, than the maximum allowed gate voltage, so that the gate oxide of the switching transistor will be overstressed, which reduces the life time of the switching transistor and therefore of the analog multiplexer as well.
Furthermore, at a supply voltage of 1.2 Volt, the operational amplifiers can only address the area between 0.2 Volt and 0.9 Volt for the analog signal. But especially in this area the switching transistors don""t conduct very well. With complementary CMOS-transistor pairs the N-channel transistor and the P-channel transistor as well conduct particularly bad, especially at half of the supply voltage (meaning 0.6 Volt).
It is the task of the invention on hand to further develop an integrated analog multiplexer of the kind previously mentioned in such a way that it is particularly well suited for low supply voltages.
According to the invention the task is solved with an integrated analog multiplexer of the previously mentioned kind in such a way that the amplifier output of the differential amplifier forms the multiplexer output, than the differential amplifier, by means of a feedback branch, is switched from the amplifier output to the inverting amplifier input as inverted amplifier and than the switch device selectively connects one of the multiplexer inputs with the inverting amplifier input.
The result is that at the multiplexer according to invention one of the analog signals that is applied to the individual multiplex inputs is switched to the inverting amplifier input (and therefore selected) in the first place and the selected signal is amplified after that. Therefore, only one differential amplifier is needed regardless of the number of multiplexer inputs, which means that the analog multiplexer can be designed in a very compact way. It is also very advantageous to first select the analog signal and to amplify it afterwards, because by this the possibly existing bad conductivity in the switch device, if it contains field effect transistor as switch elements for example, is compensated by the post-connected differential amplifier. Therefore, an integrated analog multiplexer is provided, which shows high-impedance inputs and a low-impedance output even at low supply voltages (of 1.2 Volt for example), so that even capacitive loads can be driven easily.
In particular, the switch device for each of the multiplexer inputs at the multiplexer according to invention can have a first field effect transistor of a first polarity, and it is preferred that one of the field effect transistors is closed or activated with a boosted control voltage. A boosted control voltage is a voltage that is elevated higher than a predetermined reference voltage by a so called boost-switch. Here, the boosted control voltage is preferably higher than the supply voltage of the integrated analog multiplexer. Since all first field effect transistors have the same polarity, the multiplexer according to invention can be produced especially easy and compact integrated.
The addressing of the switch device for the selective connection of one of the multiplexer inputs with the inverting amplifier input can be easily realized as well, because a sole transistor is intended for each of the multiplexer inputs and the conductivity of the activated transistors is improved because of the boosted (increased) control voltage.
Furthermore, the feedback branch can contain a second, activated field effect transistor of the first polarity, which is preferably addressed with the boosted control voltage. With the intention of a second transistor it is particularly easy to realize the same resistances in the feedback branch and in the multiplexer input channel of the multiplexer input that is connected with the inverting amplifier input, if the first and the second transistor are designed the same way.
It is preferred to use MOS-transistors as field effect transistors, since those can easily be miniaturized and are suited for small supply voltages.
Also, the switch device as well as the feedback branch can only contain field effect transistors of one polarity, so that the production of the multiplexer according to invention is simplified.
A further development of the multiplexer according to invention is that the field effect transistors are directly (without interconnection of resistors) connected with the inverting amplifier input. With that, the voltages on the source and drain ports of the transistors scarcely depend on the analog signals, so that the field effect transistors change the analog signals only slightly.
In a particularly preferred version the value of the voltage gain of the differential amplifier is one. With that, the analog multiplexer acts as a buffer, which does not change the size of the activated analog signal, except for the sign, and which has a large input impedance and an extreme small output impedance for the analog signals.
In particular at the multiplexer according to invention the non-inverting amplifier input can be set on a constant reference potential. By this, the input impedance of the analog multiplexer is essentially determined by the resistance of the multiplexer input channel through which the analog signal is connected to the inverting amplifier input.
In an advantageous advancement of the multiplexer according to invention, the switch device connects each of the multiplexer inputs that is not connected with the inverting amplifier input with a voltage source, whose potential corresponds with the potential on the inverting amplifier input. If the non-inverting amplifier input is set on the reference potential, the potential of the voltage source therefore corresponds with the reference potential, since the potential at the inverting input corresponds with the reference potential with each increase. With that, a feedback is avoided during the switching in the analog multiplexer to the sources of the analog signals, since the same potential applies all the time regardless of the condition of the switch device.
Furthermore, the differential amplifier can be supplied with a first and a second operating potential, by which the reference potential lays between the two operating potentials, preferably in the middle. With this selection of the reference potential the largest possible dynamic range of the differential amplifier can be utilized.
A particularly preferred advancement of the multiplexer according to invention is its differential design, in which the differential amplifier has a differential output with an inverting and a non-inverting amplifier output, whereby the feedback branch connects the non-inverting amplifier output with the inverting amplifier input and another feedback branch connects the inverting amplifier output with the non-inverting amplifier input and whereby each multiplexer input has a first and a second input port for a differential input signal and the switch device selectively connects the first input port of a first (selected) of the multiplexer inputs with the inverting amplifier input and the second input port of the first multiplexer input with the non-inverting amplifier input, as well as the at remaining multiplexer inputs the two input ports with each other.
It is therefore not necessary at the differential variation to place a reference potential on the non-inverting amplifier input and as a result the analog multiplexer can be designed easier and smaller. Additionally, fluctuations of the supply voltage can be better compensated due to the differential design, and the dynamic area of the difference amplifier, and therefore of the multiplexer according to invention can be doubled by the same supply voltage in comparison to the non-differential variation.
In the differential variation in particular the switch device (only) can contain field effect transistors of a polarity for switching. This eases the production of the multiplexer according to invention.
Furthermore, the switched-on field effect transistors can be addressed with a boosted control voltage, so that the analog signals can be well gated.
Furthermore, it is preferred to connect the field effect transistors directly (without interconnection of resistors) with the amplifier inputs. By that, the voltages on the source- and drain ports of the field effect transistors scarcely depend on the analog signals, by which reliability problems do not arise for the boosted transistors that are described in the description manual.
Additionally the differential amplifier at the multiplexer according to invention can be designed as an operational amplifier. By this the known advantages of an operational amplifier can be used at the analog multiplexer according to invention. Likewise, for example, the common mode rejection of the operational amplifier is extremely high and with the allocation of the operational amplifier as the inverting amplifier the output impedance is extremely small, while the input impedance is very high.